Post-shock recovery monitoring for tachyarrhythmia discrimination

ABSTRACT

A cardiac rhythm management device is configured to discriminate between ventricular and supraventricular tachycardias (referred to as SVT/VT discrimination) by utilizing a morphology criterion in which the morphology of electrogram waveforms during ventricular beats are analyzed to determine if the beats are normally conducted. After the delivery of a cardioversion/defibrillation shock, however, the intraventricular conduction system is left in a modified state which alters the subsequently generated electrogram signal. Use of the morphology criterion for SVT/VT discrimination is discontinued after delivery of such a shock and resumed after a predetermined minimum number of normally conducted ventricular beats has been detected.

FIELD OF THE INVENTION

This invention pertains to methods and apparatus for treatingarrhythmias with electrical therapy and for discriminating betweensupraventricular and ventricular tachycardias.

BACKGROUND

Tachyarrhythmias are abnormal heart rhythms characterized by a rapidrate, typically expressed in units of beats per minute (bpm), which canoriginate in either the ventricles or the atria. Examples oftachyarrhythmias include atrial tachyarrhythmias such as atrial flutterand atrial fibrillation (AF), and ventricular tachyarrhythmias such asventricular tachycardia (VT), and ventricular fibrillation (VF). Incontrast, sinus tachycardia (ST) is a benign rhythm which can alsoresult in an elevated heart rate with rates comparable to atrial andventricular tachyarrhythmias in some patients. The most dangeroustachyarrhythmias are those that have their origin in the ventricles,namely ventricular tachycardia and ventricular fibrillation. Ventricularrhythms occur when re-entry of a depolarizing wavefront in areas of theventricular myocardium with different conduction characteristics becomesself-sustaining or when an excitatory focus in the ventricle usurpscontrol of the heart rate from the normal physiological pacemaker of theheart, the sino-atrial node. The result is rapid activation of theventricles out of electromechanical synchrony with the atria. Mostventricular rhythms exhibit an abnormal QRS complex in anelectrocardiogram (ECG) because they do not use the specializedconduction system of the ventricles, the depolarization spreadinginstead from the excitatory focus or point of re-entry directly into themyocardium. In ventricular tachycardia, the ventricles activate rapidlyand produce distorted QRS complexes in an ECG. Ventricular fibrillation,on the other hand, occurs when the ventricles depolarize at an even morerapid rate and in a chaotic fashion, resulting in electrogramdeflections of constantly changing shape and virtually no effectivepumping action.

Implantable cardiac rhythm management devices may be configured to treatboth atrial and ventricular tachyarrhythmias with electrical therapy.Devices known as implantable cardioverter/defibrillators (ICDs) deliveran electric shock to the heart which terminates the tachyarrhythmia bydepolarizing all of the myocardium simultaneously and rendering itrefractory. The most dangerous tachyarrhythmias are ventriculartachycardia and ventricular fibrillation, and ICDs have most commonlybeen applied in the treatment of those conditions. Both ventriculartachycardia and ventricular fibrillation can be hemodynamicallycompromising, and both can be life-threatening. Ventricularfibrillation, however, causes circulatory arrest within seconds and isthe most common cause of sudden cardiac death and is usually treatedwith immediate delivery of a defibrillation shock. Ventriculartachycardia can be treated with either a defibrillation or acardioversion shock, the latter referring to a shock deliveredsynchronously with an R wave. Dual chamber ICDs are also capable ofdetecting atrial tachyarrhythmias, such as atrial fibrillation andatrial flutter, and delivering a cardioversion shock pulse to the atriain order to terminate the arrhythmia. Although not immediatelylife-threatening, it is important to treat atrial fibrillation forseveral reasons. First, atrial fibrillation is associated with a loss ofatrio-ventricular synchrony which can be hemodynamically compromisingand cause such symptoms as dyspnea, fatigue, vertigo, and angina. Atrialfibrillation can also predispose to strokes resulting from emboliforming in the left atrium. Although drug therapy and/or in-hospitalcardioversion are acceptable treatment modalities for atrialfibrillation, dual chamber ICDs configured to treat atrial fibrillationoffer a number of advantages to certain patients, including convenienceand greater efficacy.

Another type of electrical therapy for ventricular tachycardia isanti-tachycardia pacing (ATP). In ATP, the ventricles are competitivelypaced with one or more pacing pulses in an effort to interrupt thereentrant circuit causing the tachycardia. ATP therapy can successfullytreat VT, but it is not effective in terminating VF. Modern ICDsincorporate ATP capability so that ATP therapy can be delivered to theheart when a ventricular tachycardia is detected. Althoughcardioversion/defibrillation will also terminate ventriculartachycardia, it consumes a large amount of stored power from the batteryand results in patient discomfort owing to the high voltage of the shockpulses. It is desirable, therefore, for the ICD to use ATP to terminatea tachyarrhythmia whenever possible. In most ICDs with ATP capability,VF is distinguished from VT using a rate-based criterion so that ATP orshock therapy can be delivered as appropriate, where the heart rate isdetermined by measurement of the time interval between successiveventricular depolarizations. In a typical device, therapy delivery maybe programmably allocated into multiple rate zones, with VF therapydelivered in the highest zone, and VT therapy delivered in one or morelower rate zones. A tachyarrhythmia with a heart rate in the VT zone maybe treated with ATP therapy in order to avoid an unnecessary painfulshock to the patient, and a defibrillation shock is delivered if theheart rate is in the VF zone or if ATP pacing fails to terminate atachyarrhythmia in the VT zone.

As aforesaid, VT can be detected when the ventricular rate falls withinthe VT zone. A rapid ventricular rate in the VT zone, however, is notnecessarily due VT but can also result from a tachycardia thatoriginates from “above” the ventricles. Such tachyarrhythmias arereferred to as supraventricular tachycardias (SVT's) and include ST,which is a normal rhythm, as well as atrial tachyarrhythmias such asatrial tachycardia (AT) of non-sinus origin, atrial flutter (AFL), AVnode reentrant tachycardia (AVNRT), and atrial fibrillation (AF). Thenormal rhythmic impulse of the heart is first generated in pacemakertissue known as the sino-atrial (SA) node, spreads throughout the atriacausing atrial contraction, and is then conducted to theatrioventricular (AV) node where the impulse is delayed before passinginto the ventricles. The ventricles of a normal heart are thenelectrically stimulated by excitation emanating from the AV node thatspreads via specialized conduction pathways. An abnormal rhythm in theatria can thus be transmitted antegradely to the ventricles in a patientwhose AV conduction pathway is intact. Such an SVT is characterized byelevated rates in both the atria and the ventricles. Elevated rates inboth the atria and ventricles can also occur with VT as well, however,due to retrograde conduction of excitation from the ventricles to theatria. Such retrograde conduction is possible in most people andconfounds the discrimination between VT and SVT based upon atrial andventricular rates alone when both rates are similar, a condition knownas a one-to-one or 1:1 tachycardia.

It is desirable for an ICD to differentiate between an SVT and a VT, toensure delivery of appropriate therapy. Ventricular ATP therapydelivered to treat an SVT will not be effective and potentially couldmake matters worse by triggering a ventricular arrhythmia. Ventricularshock therapy delivered into SVTs can be perceived as painful and isineffective in reducing elevated heart rates associated with ST. It isthus important for an ICD to recognize that an elevated ventricular rateis due to an SVT rather than a VT so that ventricular ATP therapy can bewithheld and specific therapy for the atrial tachyarrhythmia can bedelivered if appropriate. Conversely, because VT is generally a moreserious condition, the ICD also needs to detect VT with a high degree ofsensitivity so that therapy can be delivered promptly. The presentinvention is directed toward improvements in methods and apparatus fordealing with this problem.

SUMMARY

A primary technique for distinguishing supraventricular tachyarrhythmiasfrom VT (referred to as SVT/VT discrimination) is by a rate based testinvolving measured atrial and/or ventricular rates. For example, a ratebased test may judge a tachyarrhythmia as a VT if the ventricular rateis greater than a specified VT threshold and also greater than theatrial rate. Additional rate based criteria such as suddenness of onsetof the fast ventricular rate, stability of the ventricular rate, andpresence of a high atrial rate above a predetermined or programmablethreshold may also be applied to distinguish VT from SVT and atrialtachyarrhythmias from ST. Discrimination between VT and SVT can be aidedby additionally employing a morphology criterion in which the morphologyof electrogram waveforms during ventricular beats are analyzed todetermine if the beats are normally conducted. A combinedmorphology/rate based test can then be used for discriminating betweenSVT and VT. An exemplary morphology/rate based algorithm judges atachyarrhythmia in which the ventricular rate is above a specified VTthreshold as an SVT only if: 1) the atrial rate is greater than or equalto the ventricular rate, and 2) some predetermined minimum number ofventricular beats has been detected as normally conducted according tothe morphology criterion, or another indication of SVT is present suchas an unstable ventricular rate in combination with a high atrial rate.

An implantable cardiac rhythm management device may treat VF and AF bythe delivery of ventricular defibrillation shocks or atrial conversionshocks. After the delivery of such a shock, however, conduction withinthe heart is left in a modified state which alters a subsequentlygenerated electrogram signal so that use of the morphology/rate testmust be temporarily discontinued until the conduction system recovers. Apost-shock recovery period may therefore be defined during which only arate based test, rather than the combined morphology/rate test, is usedfor SVT/VT discrimination. A fixed post-shock recovery period can beused so that the combined morphology/rate test is discontinued for apredetermined period of time following each shock. It is desirable,however, to resume use of the combined morphology/rate test as soon aspossible in order to permit more sensitive detection of atrialtachyarrhythmias and consequent delivery of appropriate therapy. This isespecially important in the case of atrial cardioversion shocksdelivered to terminate atrial fibrillation since early recurrence ofatrial fibrillation (ERAF) is not an uncommon event. In accordance withthe present invention, therefore, use of the morphology/rate test todistinguish between an SVT and a VT is resumed after a predeterminedminimum number of normally conducted ventricular beats has been detectedas determined by the same or a different morphology criterion used inthe morphology/rate test for SVT/VT discrimination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an exemplary physical configuration of an implantedcardiac rhythm management device.

FIG. 2 is a system diagram of an implantable cardiac rhythm managementdevice.

FIG. 3 illustrates an exemplary algorithm for post-shock recoverymonitoring and resuming use of a morphology criterion fortachyarrhythmia discrimination following delivery of an atrial orventricular cardioversion shock for a dual chamber defibrillator.

FIG. 4 illustrates an exemplary algorithm for post-shock recoverymonitoring and resuming use of a morphology criterion fortachyarrhythmia discrimination following delivery of a ventricularcardioversion shock for a ventricular defibrillator.

FIG. 5 illustrates a truth table of an exemplary method for combiningmorphology and rate based criteria yielding an overall SVT/VTdiscrimination decision for dual chamber defibrillators and ventriculardefibrillators with dual chamber sensing and pacing.

FIG. 6 illustrates a truth table of an exemplary method for combiningmorphology and rate based criteria yielding an overall SVT/VTdiscrimination decision for ventricular defibrillators with singlechamber (ventricular only) sensing and pacing

DETAILED DESCRIPTION

As noted above, ICDs may be configured for delivering ventricularanti-tachycardia pacing or shocks in order to terminate ventriculartachycardias and for delivering atrial anti-tachycardia pacing or atrialconversion shocks in order to terminate atrial tachyarrhythmias, and itis important for such devices be able to distinguish between an SVT andVT. One way by which a device may distinguish between an SVT and VT isto employ a rate based test so that if a ventricular rate is detectedthat is high enough to constitute a VT, the arrhythmia is categoricallyclassified as a VT if the ventricular rate is also greater than theatrial rate. A tachycardia in which the atrial rate is greater than orequal to the ventricular rate, on the other hand, may be a VT, an SVT,or a dual tachycardia where both VT and SVT are present simultaneously.Since VT is the more serious condition and requires prompt treatment,the rate based test may require that one or more additional rate basedcriteria be met before VT is ruled out. Such criteria, which helpidentify SVT, may include instability of the ventricular rate and anatrial rate above a specified atrial tachyarrhythmia threshold. Only ifthese one or more additional criteria are met is the tachycardia thenclassified as an SVT. The atrial tachyarrhythmia threshold may also beused to further classify the SVT as an atrial tachyarrhythmia requiringtreatment, such as atrial fibrillation, or as ST which is not consideredan arrhythmia and requires no treatment.

In order to more accurately detect an SVT, a combined morphology/ratebased test may be used instead of the rate based test described abovefor SVT/VT discrimination. In a combined morphology/rate based test, amorphology criterion based upon a morphology analysis of electrogramwaveforms is used to provide an additional criterion for detecting anSVT. For example, SVT may be detected if: 1) the atrial rate is greaterthan or equal to the ventricular rate and, 2) some minimum number ofventricular beats are judged as normally conducted as determined by amorphology criterion involving morphology analyses of electrogramwaveforms. An exemplary implementation of a morphology criterion forSVT/VT discrimination is disclosed in U.S. Pat. No. 6,449,503, assignedto Cardiac Pacemakers, Inc., the disclosure of which is herebyincorporated by reference in its entirety.

A morphology analysis may be performed for purposes of implementing amorphology criterion in different ways, such as by identifyingparticular features in the an electrogram signal generated during aparticular ventricular beat and by comparing an electrogram signalgenerated during a particular ventricular beat with a reference templaterepresenting a normally conducted ventricular beat. One means by whichan electrogram signal may be compared with a template is by performing across-correlation between the electrogram signal and the template. Ifthe beat electrogram signal and the reference template are sufficientlycorrelated, the ventricular beat can then be judged as normallyconducted. The comparison of the beat electrogram waveform and thereference template waveform may be performed by computing a featurecorrelation coefficient between selected corresponding features of thetwo waveforms where the selected features may represent amplitudes ofthe electrogram waveform at various points. An exemplary formula forcomputing a feature correlation coefficient (FCC) is given by:FCC=(NΣX _(i) Y _(i)−(ΣX _(i))(ΣY _(i)))²/(NΣX _(i) ²−(ΣX _(i))²)(NΣY_(i) ²−(ΣY _(i))²)where X_(i) represent beat features, Y_(i) represent template features,N is the number of features, and the summations are carried out from ito N. In a particular embodiment of a morphology criterion, N is equalto 8, and a ventricular beat is judged as normally conducted if FCC isgreater than 0.94. In order for the beat morphology to be correlated tothe template as opposed to inversely correlated with an invertedmorphology, the following additional condition must be met:NΣX _(i) Y _(i)−(ΣX _(i))(ΣY _(i))>0

Morphology discrimination is possible from the ventricular pacingelectrodes. However, the beat electrogram signal for morphology analysisis preferably obtained from electrodes which record a signal thatcaptures overall ventricular dynamics. Such electrodes may be describedas having a “unipolar” configuration in that several centimetersseparate the electrodes. “Unipolar” electrodes positioned so that alarge portion of the heart falls between them “see” a larger volume ofthe myocardium, and changes in the depolarization pattern of theventricles will be more readily reflected in an electrogram generated bythe electrode during a ventricular beat. A convenient electrode for thispurpose is the shock electrode that the device normally uses fordelivering cardioversion/defibrillation shocks. The sensing channelincorporating the shock electrode and which is used to generateelectrograms for morphology analysis is referred to herein as the shockchannel. It should be appreciated that a sensing channel for generatingelectrograms suitable for morphology analysis could use an electrodeother than the shock electrode. In the description that follows, theterm shock channel should be taken to mean any sensing channel used togenerate electrograms for morphology analysis, regardless as to whethera shock electrode is actually used.

In order to ensure that corresponding features of the beat and templatewaveforms are used in computing the feature correlation coefficient, thetwo waveforms must be aligned with respect to some temporal referencepoint. Such alignment may be accomplished by recording a bipolarelectrogram using a sensing channel normally used to sense R waves forrate determination, referred to as the rate channel, simultaneously withthe shock channel electrogram used as the beat electrogram. Thereference template electrogram is then aligned with respect to aselected alignment point of the rate channel electrogram (the peakamplitude of the QRS complex). Amplitudes of the shock channelelectrogram and the reference template occurring at predetermined timeswith respect to the alignment point are then used as the features forcomputing the feature correlation coefficient.

Immediately following delivery of a cardioversion/defibrillation shock,however, morphology analysis of electrogram waveforms is no longer ableto reliably identify normally conducted beats. The reason for this isthat, after a shock pulse is output from electrodes, the electricalconduction within the heart is temporarily modified, so that conductionof supraventricular beats proceeds through the ventricular myocardiumwith a different vector until conduction recovers, resulting in adepolarization waveform with altered morphology and which may haveinverted polarity. Use of the combined morphology/rate test for SVT/VTdiscrimination should therefore be discontinued after acardioversion/defibrillation shock is delivered until the conductionsystem recovers. Following delivery of an atrial conversion orventricular defibrillation/cardioversion shock, the device couldinitiate a fixed post-shock recovery period, during which only a ratebased test is used for SVT/VT discrimination so that a ventricular ratein the VT zone which is greater than or equal to the atrial rate isdetected as a VT. It is desirable to resume use of the morphology/ratebased test as soon as possible, however, because all 1:1 tachycardiasand some other SVT's will be interpreted by the rate based test as VT.This increases the probability that inappropriate therapy (i.e.,ventricular anti-tachycardia pacing or shock) will be delivered to treatan SVT. This situation is particularly likely to occur followingdelivery of an atrial conversion shock to treat AF due to the phenomenonof early recurrence of atrial fibrillation or ERAF. ERAF is defined asthe recurrence of atrial fibrillation within a few minutes aftersuccessful cardioversion with atrial shock therapy, and certain patientsare more prone to experience it than others.

In accordance with the present invention, a cardiac rhythm managementdevice is configured to generate an electrogram signal from a cardiacsensing channel and programmed to utilize a morphology criterion derivedfrom analysis of the electrogram signal for SVT/VT discrimination. Thedevice is further programmed to discontinue use of the morphologycriterion for SVT/VT discrimination after delivery of acardioversion/defibrillation shock and to resume use of the morphologycriterion for SVT/VT discrimination when a predetermined minimum numberof ventricular beats has been judged as normally conducted. Ventricularbeats may be judged as normally conducted through the use of ratemeasurements, morphology analysis, or both.

In an exemplary embodiment, a dual-chamber or single-chamberdefibrillator is configured to discontinue use of a morphology criterionin a combined morphology/rate based test for SVT/VT discriminationimmediately following delivery of a cardioversion/defibrillation shock.In order to minimize the time period following the shock during whichthe morphology criterion is not used for SVT/VT discrimination, thedevice is further configured to monitor the electrogram signalsgenerated by the shock channel during the normal sinus rhythm followingsuccessful termination of the atrial or ventricular tachyarrhythmia.Each shock channel electrogram can then be judged as normally conductedor not using the same or a different morphology criterion used by themorphology/rate based test in distinguishing between an SVT and a VT.When normal sinus rhythm is present (as determined by measurement of theatrial and/or ventricular rates), all of the ventricular beats can beassumed to be normally conducted (i.e., supraventricular rhythm ispresent). Therefore, detection of a normally conducted beat from theshock channel electrogram during the post-shock period implies that theintra-ventricular conduction system has recovered to its normal stateand that electrograms will exhibit normal depolarization morphology. Inaccordance with the invention, the device is configured to resume use ofthe morphology criterion as part of a combined morphology/rate basedtest to distinguish between an SVT and a VT after a predeterminedminimum number of normally conducted ventricular beats has beendetected. A particular embodiment of the invention is described in moredetail below.

1. Exemplary Device Description

Cardiac rhythm management devices such as ICDs and pacemakers aretypically implanted subcutaneously on a patient's chest and have leadsthreaded intravenously into the heart to connect the device toelectrodes used for sensing cardiac activity, delivering pacing pulses,and/or delivering defibrillation shocks. FIG. 1 depicts an implantablecardioverter/defibrillator device for treating atrial and ventriculartachyarrhythmias that also incorporates functionality for pacing theatria and/or the ventricles. The device includes a subcutaneouslyimplantable housing or can 60 for enclosing the electronic circuitry ofthe device and a pair of leads L1 and L2 having electrodes incorporatedtherein. The lead L1 has a tip electrode 33 a and ring electrode 33 bwhich are shown in the figure as disposed in the right atrium (RA) forpacing or sensing of the atria. The lead. L2 has a tip electrode 43 a, adistal coil electrode 43 b, and a proximal coil electrode 43 c. In theplacement of the lead L2 shown in the figure, tip electrode 43 a anddistal coil electrode 43 b are disposed in the right ventricle (RV), andproximal coil electrode 43 c is disposed in the superior vena cava orright atrium. Sensing or pacing of the ventricles may be performed, e.g.using tip electrode 43 a as the cathode and coil electrode 43 b as theanode. A ventricular cardioversion/defibrillation shock or an atrialconversion shock may be delivered between coil 43 b and the can 60,between the coil 43 b and coil 43 c, or between coil 43 b and the can 60electrically in common with the coil 43 c.

FIG. 2 is a system diagram of the implantable device shown in FIG. 1.The controller of the device is made up of a microprocessor 10communicating with a memory 12, where the memory 12 may comprise a ROM(read-only memory) for program storage and a RAM (random-access memory)for data storage and additional program storage. A microprocessor-typecontroller 10 controls the overall operation of the device in accordancewith programmed instructions stored in memory. The controller could beimplemented by other types of logic circuitry (e.g., discrete componentsor programmable logic arrays) using a state machine type of design, buta microprocessor-based system is preferable. As used herein, the term“circuitry” should be taken to refer to either discrete logic circuitryor to the programming of a microprocessor. A telemetry interface 80 isprovided for communicating with an external programmer 300. The externalprogrammer is a computerized device with a controller 330 that caninterrogate the device and receive stored data as well as adjust thedevice's operating parameters. The device is equipped with multiplesensing amplifiers and pulse generators which can be configured aschannels for pacing and/or sensing selected heart chambers. A switchmatrix 70 controlled by the microprocessor is used to configure asensing or pacing channel by switching selected electrodes to the inputof a sense amplifier or to the output of a pulse generator. The switchmatrix 70 allows the device to employ either bipolar sensing/pacingusing two closely spaced electrodes of a lead or unipolar sensing/pacingusing one of the electrodes of a lead and the can 60 as a referenceelectrode. The switch matrix 70 can connect shock generator 85 todeliver a ventricular cardioversion/defibrillation shock or atrialconversion shock between coil electrode 43 b and the can 60 (or the coilelectrode 43 c, or the can 60 connected in common with the coilelectrode 43 c). In the device shown in FIG. 2, an atrial channel forsensing or pacing an atrial site is configured with tip electrode 33 a,ring electrode 33 b, sense amplifier 31, pulse generator 32, and anatrial channel interface 30 which communicates bidirectionally with aport of microprocessor 10. A first ventricular channel for sensing orpacing a ventricular site is configured with tip electrode 43 a, coilelectrode 43 b, sense amplifier 41, pulse generator 42, and ventricularchannel interface 40. This channel may be used as the rate channelreferred to earlier which generates an electrogram for aligning thereference template waveform. A second ventricular sensing channel usingventricular channel interface 50 may be configured by connecting one ofthe differential inputs of sense amplifier 51 to the coil electrode 43 band connecting the other input to the can 60 and coil electrode 43 c.This is the shock channel for generating the beat electrogram used formorphology analysis as described above.

The channel interfaces may include comparators for comparing receivedelectrogram signals to reference values, analog-to-digital convertersfor digitizing sensing signal inputs from the sensing amplifiers,registers that can be written to for adjusting the gain and sensingthreshold values of the sensing amplifiers, and registers forcontrolling the output of pacing pulses and/or adjusting the pacingpulse energy by changing the pulse amplitude or pulse width. Thecontroller uses the sensing channels in order to detect intrinsiccardiac activity in a heart chamber, referred to as a chamber sense(e.g., an atrial sense or a ventricular sense). In order to detectintrinsic cardiac activity, the signals emanating from the senseamplifier are compared with a reference potential. As described above, asensing channel includes sense amplifier circuits for amplifying andfiltering the electrogram signals picked up by an electrode disposed ata cardiac site. Only when an electrogram signal from the sense amplifierexceeds a reference potential, referred to as a sensing threshold, is ittreated as a chamber sense. The sensing threshold may be implementedwith analog circuitry, where the sense amplifier output is applied toone input of a comparator circuit whose other input is connected to areference potential, or with digital circuitry operating on digitizedsamples of the sense amplifier output which are compared with adigitized reference value. In either case, the sensing threshold foreach channel is adjustable by the controller. Detected chamber sensesmay be used for controlling the delivery of paces in accordance with aprogrammed pacing mode (e.g., ventricular anti-tachycardia pacing)and/or for diagnostic purposes. By counting the number of chamber sensesover a defined time period, the controller is able to measure heart rateand detect arrhythmias using rate-based criteria. The atrial sensingchannel and the first ventricular sensing channel described above areused to separately measure the atrial and ventricular rates in thisembodiment.

When the measured atrial and/or ventricular rates exceed specifiedthreshold values, the device detects an arrhythmia and is programmed torespond with appropriate therapy. For example, if a ventricular rate ismeasured which is in the VF zone, the device delivers a ventriculardefibrillation shock. If a ventricular rate is measured which is in theVT zone, the device decides whether VT or an SVT is present using therate and morphology criteria described above. If the ventricular rate isgreater than the atrial rate, VT is detected, and the device may beprogrammed to initiate ventricular anti-tachycardia pacing or aventricular shock. If the atrial rate is greater than or equal to theventricular rate and a specified minimum number of normally conductedbeats are detected, an SVT is detected, and, if the SVT is classified asan atrial tachyarrhythmia, the device may be programmed to deliveratrial anti-tachycardia pacing or an atrial cardioversion shock. Ratestability, suddenness of onset, and/or atrial rate criteria can also beused as additional criteria to classify the arrhythmia as atrial inorigin when the ventricular rate is in the VT zone. A dual chamberdefibrillator, such as illustrated by FIG. 2, would also detect anatrial tachyarrhythmia and deliver an atrial anti-tachycardia pacing oran atrial cardioversion shock if the atrial rate is above a specifiedthreshold value and the ventricular rate is in the normal range, ascould occur in a patient without an intact AV conduction pathway. Tolessen the risk of inducing a ventricular arrhythmia, the device maydeliver the atrial cardioversion shock synchronously with a sensedventricular depolarization (i.e., an R wave) and may delay deliveringthe shock until the interval from the previous ventricular beat meetscertain criteria, e.g. is longer than a predetermined or programmablethreshold. Methods for R-wave synchronization are described in U.S. Pat.No. 5,207,219, hereby incorporated by reference.

The device illustrated in FIG. 2 is a dual-chamber defibrillator havingthe capability of sensing both the atria and the ventricles and ofdelivering shock and/or ATP therapy to both the atria and theventricles. Various embodiments of the invention may be incorporatedinto devices possessing either some or all of the components illustratedin FIG. 2. For example, the post-shock recovery monitoring algorithms inaccordance with the invention may be incorporated into a dual-chamberdefibrillator having atrial and ventricular sensing capability, asingle-chamber defibrillator having atrial and ventricular sensingcapability, or a single-chamber defibrillator having only ventricularsensing capability.

2. Exemplary Post-Shock Recovery Monitoring Algorithms

In an exemplary embodiment of the invention, a cardiac rhythm managementdevice such as depicted in FIG. 2 is configured to distinguish betweenVT and SVT by using a combined morphology/rate test as described above.To perform the morphology analysis, the beat electrogram is correlatedwith a template to determine if a beat is normally conducted. The beatelectrogram is obtained from a shock channel incorporating the shockelectrode used to deliver cardioversion and/or defibrillation shocks.The device distinguishes between SVT and VT with a combinedmorphology/rate based test which includes: a) a rate criterion involvingthe measured atrial and ventricular rates, and b) a morphologycriterion, wherein a first predetermined minimum number of beats must bejudged as normally conducted ventricular beats before an SVT isdetected. After detecting a tachyarrhythmia and subsequently restoringnormal sinus rhythm with an atrial cardioversion or ventricularcardioversion/defibrillation shock delivered through the shockelectrode, the device discontinues use of the combined morphology/ratebased test to distinguish between an SVT and a VT and employs a ratebased test instead. The use of the combined morphology/rate based testis then resumed only when a second predetermined minimum numberventricular beats during supraventricular rhythm has been judged asnormally conducted.

FIG. 5 is a truth table representation of a combined rate/morphologybased test for SVT/VT discrimination utilizing measurements of both theatrial and ventricular rates. Such a test could be implemented by a dualchamber defibrillator or a ventricular defibrillator with both atrialand ventricular sensing capability.

FIG. 3 illustrates an exemplary algorithm, as would be implemented byappropriate programming of the controller, by which a dual chamberdefibrillator utilizing both a rate based test and a combinedmorphology/rate based test for SVT/VT discrimination may performpost-shock recovery monitoring in order to determine when a validelectrogram for morphology analysis can be used after a shock isdelivered. In this embodiment, both a rate based test and a combinedmorphology/rate based test are used for SVT/VT discrimination. In anexemplary rate based test, a tachycardia is classified as an SVT if theatrial rate is greater than a preset threshold (e.g., 200 bpm), theatrial rate is greater than or equal to the ventricular rate, and theventricular rate is unstable. Instability of the ventricular rate may bedetermined by measuring the differences between successive RR intervals(i.e., intervals between ventricular senses), computing an average ofthose differences, comparing the average to a preset threshold (e.g., 20ms), and deeming the ventricular rate to be unstable if the averageexceeds the threshold. In an alternate embodiment of the rate basedtest, the test may also judge a tachycardia as a VT if the ventricularrate is greater than the atrial rate or the ventricular rate is suddenlyincreased and stable. This is useful in distinguishing VT which usuallystarts with a sudden increase in rate from sinus tachycardia whichusually begins with a gradual increase in rate. In an exemplary combinedmorphology/rate based test utilizing measurements of both atrial andventricular rates, as illustrated by FIG. 5 in the form of a truthtable, a tachyarrhythmia is judged as an SVT if: 1) the measured atrialrate is greater than or equal to the measured ventricular rate and 2) apredetermined minimum fraction of a specified number of ventricularbeats has been judged as normally conducted or the combined conditionsof high atrial rate (e.g. >200 bpm) and ventricular rate instability arepresent.

Referring to FIG. 3, atrial and ventricular rates are measured fromsensed atrial and ventricular electrical activity at step S1 in order todetect atrial and ventricular tachyarrhythmias. If no ventriculararrhythmia is detected at step S2, the presence of an atrialtachyarrhythmia is checked at step S16 as discussed below. If aventricular arrhythmia is detected at step S2 because the ventricularrate is above a specified threshold, the device next determines whattype of arrhythmia is occurring in order to deliver appropriate therapy.If the arrhythmia is in the VF zone as determined at step S3, adefibrillation shock is delivered and a post-shock recovery flag is setat step S4. Otherwise, the arrhythmia may either be VT or SVT such asatrial fibrillation. At step S5, it is determined whether the post-shockrecovery flag is set or cleared. If the flag is set, only a rate basedtest is used for SVT/VT discrimination at step S6. If, on the otherhand, the post-shock recovery flag is cleared, the algorithm employs acombined morphology/rate based test for SVT/VT discrimination at stepS7. An electrogram signal is generated from the shock electrode, and afeature correlation coefficient is computed for selected ventricularbeats. The feature correlation coefficient reflects the extent ofcorrelation between a reference template representing normal ventricularconduction and the electrogram signal generated from the shockelectrode, and a ventricular beat is judged as normally conducted if thefeature correlation coefficient exceeds a predetermined threshold value(e.g., 94%). An SVT and a VT are distinguished according to the combinedmorphology/rate based test which detects an arrhythmia as an SVT if: 1)the measured atrial rate is greater than or equal to the measuredventricular rate and some predetermined minimum fraction of a specifiednumber of ventricular beats has be judged as normally conducted, or 2)if the atrial rate is greater than a preset threshold, the atrial rateis greater than or equal to the ventricular rate, and the ventricularrate is unstable. If a ventricular tachycardia is detected by either therate based test or the morphology/rate based test at step S8, VT therapyis delivered at step S9, and the algorithm returns to step S1. If no VTis detected at step S8, the atrial rate is evaluated at step S16 todetermine if an atrial tachyarrhythmia is present. (In the case of adevice with no atrial therapy capability, the algorithm would return tostep S1 if no VT is detected at step S8.) If an atrial tachyarrhythmiais not detected at step S16, then the algorithm returns to step S1. Ifan atrial tachyarrhythmia is detected, an atrial therapy is delivered atstep S10. If a shock is delivered at steps S9 or S10, then thepost-shock recovery flag is set. The algorithm then returns to step S1and, concurrently, a post-shock recovery monitoring routine begins atstep S11. At step S12, the device determines if supraventricular rhythmexists. When supraventricular rhythm is present, the device proceeds tostep S13 where the morphology of one or more beat electrograms areanalyzed using the same or different morphology criterion used indistinguishing between an atrial tachyarrhythmia and a ventriculartachycardia. After a predetermined minimum number of normally conductedventricular beats has been detected at step S14, the post-shock recoveryflag is cleared at step S15.

FIG. 4 illustrates an exemplary algorithm for a single chamberdefibrillator, as would be implemented by appropriate programming of thecontroller, by which the device may perform post-shock recoverymonitoring in order to determine when a valid electrogram for morphologyanalysis can be used after a shock is delivered. This algorithm issimilar to that of the dual chamber defibrillator illustrated by FIG. 3,but does not provide atrial defibrillation shock or atrial ATP andutilizes ventricular sensing only. Thus, only the ventricular rate ismeasured at S1, and steps S10 and S16 of FIG. 3 are omitted in FIG. 4.As in the above-described dual chamber embodiment, both a rate basedtest and a combined morphology/rate based test are used for SVT/VTdiscrimination. These tests may be similar to those previously describedfor the dual chamber defibrillator algorithm except that no measurementsof the atrial rate are used. For example, the rate based test may judgea tachycardia as a VT rather than an SVT if the ventricular rate issuddenly increased and stable.

A truth table for an exemplary combined morphology/rate based test forSVT/VT discrimination with single chamber (ventricular only) sensing isshown in FIG. 6. This combined morphology/rate based test employs thecriteria of the rate based test so that a tachyarrhythmia is judged asan SVT if the ventricular rate is unstable. The addition of theventricular stability criterion in the single chamber case allows theoption of detecting AF with significant morphological variation ifventricular instability is present. If the ventricular stabilitycriterion is used in the single chamber defibrillator with singlechamber sensing, the VT zone should be selected to be below thepatient's polymorphic VT rate, since polymorphic VT also exhibits anunstable ventricular response.

In an alternative embodiment, a plurality of electrograms generated bythe shock electrode are continuously stored in a rolling onset buffer.Upon detection of a tachyarrhythmia, feature correlation coefficientsare computed for the electrograms stored in the onset buffer, and it isdetermined whether the stored electrograms indicate normally conductedventricular beats. The combined morphology/rate based test is then usedto distinguish between an atrial tachyarrhythmia and a ventriculartachycardia only if a predetermined minimum number of detected eventsassociated with normally conducted ventricular beats are stored in theonset buffer.

Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

1. A method for operating a cardiac rhythm management device, comprising: generating a first electrogram signal from a cardiac sensing channel; utilizing a morphology criterion derived from analysis of the first electrogram signal for discriminating between a supraventricular tachycardia (SVT) and a ventricular tachycardia (VT), referred to as SVT/VT discrimination; discontinuing use of the morphology criterion for SVT/VT discrimination after delivery of a cardioversion/defibrillation shock; and, resuming use of the morphology criterion for SVT/VT discrimination when a first predetermined minimum number of ventricular beats has been judged as normally conducted.
 2. The method of claim 1 wherein ventricular beats are judged as normally conducted by utilizing a morphology criterion.
 3. The method of claim 1 further comprising: measuring atrial and ventricular rates from sensed atrial and ventricular electrical activity; computing a feature correlation coefficient for selected ventricular beats, the feature correlation coefficient reflecting the extent of correlation between a reference template representing normal ventricular conduction and the first electrogram signal, and judging a ventricular beat as normally conducted if the feature correlation coefficient exceeds a predetermined threshold value; discriminating between SVT and VT by a combined morphology/rate based test which includes: a) a rate criterion involving the measured atrial and ventricular rates, and b) a morphology criterion, wherein a second predetermined minimum number of normally conducted ventricular beats must be detected during a tachyarrhythmia before an SVT is detected; detecting a tachyarrhythmia and subsequently restoring normal sinus rhythm with a cardioversion/defibrillation shock; discontinuing use of the morphology/rate based test for SVT/VT discrimination after delivery of the shock and substituting a rate based test therefor; and, resuming use of the morphology/rate based test for SVT/VT discrimination when the first predetermined minimum number of ventricular beats has been judged as normally conducted following the cardioversion/defibrillation shock.
 4. The method of claim 3 further comprising: setting a post-shock recovery flag after delivery of a shock; if the post-shock recovery flag is set, employing the rate based test for SVT/VT discrimination; if the post-shock recovery flag is cleared, employing the combined morphology/rate based test for SVT/VT discrimination where a tachycardia slower than VF zone is deemed to be SVT if: 1) the measured atrial rate is greater than or equal to the measured ventricular rate, and a predetermined minimum fraction of a specified number of ventricular beats has been judged as normally conducted or 2) if the atrial rate is greater than a preset threshold, the atrial rate is greater than or equal to the ventricular rate, and the ventricular rate is unstable; monitoring ventricular beats during supraventricular rhythm after delivery of a shock and clearing the post-shock recovery flag when the first predetermined minimum number of ventricular beats has been judged as normally conducted.
 5. The method of claim 4 further comprising beginning to count normally conducted ventricular beats after a predetermined delay period following delivery of a shock.
 6. The method of claim 4 wherein the rate based test judges a tachycardia as a ventricular tachycardia if the ventricular rate is greater than the atrial rate or the ventricular rate is suddenly increased and stable.
 7. The method of claim 4 wherein the rate based test judges a tachycardia as an SVT if the atrial rate is greater than a preset threshold, the atrial rate is greater than or equal to the ventricular rate, and the ventricular rate is unstable.
 8. The method of claim 5 wherein the predetermined delay period is approximately 10 seconds.
 9. The method of claim 1 further comprising resuming use of the morphology criterion following a shock if at least one ventricular beat has been judged as normally conducted.
 10. The method of claim 1 further comprising periodically testing for a normally conducted ventricular beat following a shock and, if a normally conducted ventricular beat is detected, beginning to count normally conducted ventricular beats and resuming use of the morphology criterion if the first predetermined minimum number of normally conducted ventricular beats are counted.
 11. The method of claim 1 wherein the first predetermined number of normally conducted beats is a specified minimum fraction of a plurality of ventricular beats which are tested for normal conduction following a shock.
 12. The method of claim 1 further comprising discontinuing use of the morphology criterion SVT/VT discrimination after delivery of a shock for a predetermined length of time and resuming use of the combined morphology/rate based test with a lowered threshold value for judging a ventricular beat as being normally conducted if the first predetermined minimum number of ventricular beats judged as normally conducted has not yet been counted.
 13. The method of claim 2 further comprising: generating a second electrogram from a bipolar electrode; computing the feature correlation coefficient for a particular ventricular beat by aligning the first electrogram and the normal conduction template with the second electrogram and computing a correlation coefficient between selected points of the first electrogram and the normal conduction template.
 14. The method of claim 1 wherein the shock delivered to terminate a tachyarrhythmia is an atrial conversion shock if an atrial tachyarrhythmia is detected and a ventricular cardioversion/defibrillation shock if a ventricular tachycardia is detected.
 15. The method of claim 1 further comprising: measuring a ventricular rate from sensed ventricular electrical activity; computing a feature correlation coefficient for selected ventricular beats, the feature correlation coefficient reflecting the extent of correlation between a reference template representing normal ventricular conduction and the first electrogram signal, and judging a ventricular beat as normally conducted if the feature correlation coefficient exceeds a predetermined threshold value; discriminating between SVT and VT by a combined morphology/rate based test which includes: a) a rate criterion involving the measured ventricular rate, and b) a morphology criterion, wherein a second predetermined minimum number of normally conducted ventricular beats must be detected during a tachyarrhythmia before an SVT is detected; detecting a tachyarrhythmia and subsequently restoring normal sinus rhythm with a cardioversion/defibrillation shock; discontinuing use of the morphology/rate based test for SVT/VT discrimination after delivery of the shock and substituting a rate based test therefor; and, resuming use of the morphology/rate based test for SVT/VT discrimination when the first predetermined minimum number of ventricular beats has been judged as normally conducted following the cardioversion/defibrillation shock.
 16. A method for operating a cardiac rhythm management device, comprising: measuring a ventricular rate from sensed ventricular electrical activity; delivering a cardioversion/defibrillation shock upon detection of a tachyarrhythmia; generating an electrogram signal from a cardiac sensing channel electrodes; computing a feature correlation coefficient for selected ventricular beats, the feature correlation coefficient reflecting the extent of correlation between a reference template representing normal ventricular conduction and the electrogram signal, and judging a ventricular beat as normally conducted if the feature correlation coefficient exceeds a predetermined threshold value; discriminating between SVT and VT by a combined morphology/rate based test which includes: a) a rate criterion based upon the measured ventricular rate, and b) a morphology criterion, wherein a first predetermined minimum number of normally conducted ventricular beats must be detected during a tachyarrhythmia before an SVT is detected; continuously storing a plurality of electrograms generated by the shock electrode in a rolling onset buffer; and, upon detection of a tachyarrhythmia, computing feature correlation coefficients for the electrograms stored in the onset buffer, determining whether the stored electrograms indicate normally conducted ventricular beats, and employing the combined morphology/rate based test to discriminate between SVT and VT only if a second predetermined minimum number of electrograms indicating normally conducted ventricular beats are stored in the onset buffer.
 17. The method of claim 16 further comprising: measuring an atrial rate from sensed atrial activity; and, wherein the rate criterion is based upon the measured atrial and ventricular rates.
 18. A cardiac rhythm management device, comprising: means for delivering a cardioversion/defibrillation shock; means for generating a first electrogram signal; means for utilizing a morphology criterion derived from analysis of the first electrogram signal for discriminating between a supraventricular tachycardia (SVT) and a ventricular tachycardia (VT), referred to as SVT/VT discrimination; means for discontinuing use of the morphology criterion for SVT/VT discrimination after delivery of a cardioversion/defibrillation shock; and, means for resuming use of the morphology criterion for SVT/VT discrimination when a first predetermined minimum number of ventricular beats has been judged as normally conducted.
 19. The device of claim 18 wherein ventricular beats are judged as normally conducted by utilizing a morphology criterion.
 20. The device of claim 18 further comprising: means for measuring a ventricular rate from sensed ventricular electrical activity; means for computing a feature correlation coefficient for selected ventricular beats, the feature correlation coefficient reflecting the extent of correlation between a reference template representing normal ventricular conduction and the first electrogram signal, and judging a ventricular beat as normally conducted if the feature correlation coefficient exceeds a predetermined threshold value; means for discriminating between SVT and VT by a combined morphology/rate based test which includes: a) a rate criterion involving the measured ventricular rate, and b) a morphology criterion, wherein a second predetermined minimum number of normally conducted ventricular beats must be detected during a tachyarrhythmia before an SVT is detected; means for detecting a tachyarrhythmia and subsequently restoring normal sinus rhythm with a cardioversion/defibrillation shock; means for discontinuing use of the morphology/rate based test for SVT/VT discrimination after delivery of the shock and substituting a rate based test therefor; and, means for resuming use of the morphology/rate based test for SVT/VT discrimination when the first predetermined minimum number of ventricular beats has been judged as normally conducted during the normal sinus rhythm following the cardioversion/defibrillation shock.
 21. The device of claim 20 further comprising: means for measuring an atrial rate from sensed atrial activity; and, wherein the rate criterion is based upon the measured atrial and ventricular rates.
 22. The device of claim 21 wherein the cardioversion/defibrillation shock delivering means includes means for delivering shocks to the ventricles and means for delivering shocks to the atria.
 23. The device of claim 21 further comprising: means for setting a post-shock recovery flag after delivery of a shock; means for employing the rate based test to distinguish between an atrial tachyarrhythmia and a ventricular tachycardia if the post-shock recovery flag is set; means for employing the combined morphology/rate based test for SVT/VT discrimination if the post-shock recovery flag is cleared, where a tachycardia is judged as an SVT if: 1) the measured atrial rate is greater than or equal to the measured ventricular rate, and 2) a predetermined minimum fraction of a specified number of ventricular beats has been judged as normally conducted; means for monitoring ventricular beats during supraventricular rhythm after delivery of a cardioversion/defibrillation shock and clearing the post-shock recovery flag when the first predetermined minimum number of ventricular beats has been judged as normally conducted.
 24. The device of claim 23 further comprising means for beginning to count normally conducted ventricular beats after a predetermined delay period following delivery of a shock. 